Ultrasonic welding

Solid state: fast vibration bonds small joints in under a second

Ultrasonic welding joins parts by pressing them together and shaking them very fast, about 20,000 to 40,000 times a second. The rubbing heats the joint until it bonds, in well under a second, with no glue, screws or filler. Most welded plastic housings, from car lights to toothbrush handles, and many battery and wire connections are made this way.

Photo: Herrmann Ultraschalltechnik, CC BY-SA 4.0, via Wikimedia Commons

Ultrasonic welding · welding stack and a close-up of the jointCircled numbers match the steps below; the close-up on the right shows what happens at the joint in plastic parts.

How it works

Load the parts. The two parts sit in a fixture on the anvil (a fixed support shaped to hold them). Plastic parts are molded with a small ridge called an energy director where they touch.
Press. The horn (also called a sonotrode, the shaped metal tip of the machine) comes down and squeezes the parts together.
Vibrate. A generator sends high-frequency current to the converter, whose piezoelectric crystals (materials that change shape with voltage) turn it into vibration. The booster and horn amplify it to a movement of a few hundredths of a millimeter.
Melt the joint. Friction concentrates heat at the small contact point, so the energy director melts first and flows across the joint. The rest of the part stays cool.
Hold and cool. The vibration stops, but the force stays on for a moment while the melt freezes. The two parts are now one.

Metals work differently. For metal wires, foils and battery tabs, the horn scrubs sideways instead of hammering up and down. The rubbing breaks up surface oxides and presses clean metal together, so the joint forms without melting.

Key facts

Materials: Thermoplastics such as ABS, polycarbonate, polypropylene and nylon; thin, soft metals such as copper and aluminum foils and wires
Typical parts: Car light housings, medical devices, filters, toys, blister packs, battery tabs, wire harness splices
Part size: Joint areas from a few millimeters up to about 20–30 cm with one horn; metal sheets usually under about 1 mm
Precision: Very repeatable; height of the welded part typically within about ±0.05–0.1 mm when the machine controls the collapse
Surface finish: Clean, often invisible joint; sometimes a small bead of squeezed-out plastic (flash) or a textured mark from the horn on metals
Batch size it suits: Large series; each part shape needs its own horn and fixture
Tooling and machine cost: Tens of thousands of dollars for a welder; a few thousand for each custom horn and fixture
Speed: Weld time typically 0.1–1 s; a full cycle of a few seconds, so thousands of parts a day per machine

Strengths and limits

Strengths

Very fast, clean and easy to automate
No glue, screws, solvents or filler to buy
Low energy use, heat only at the joint
Joins dissimilar metals such as copper to aluminum

Limits

Only for small joints and fairly thin parts
Each part design needs its own horn and fixture
Works only with thermoplastics and soft, thin metals
Vibration can damage delicate electronics inside the part

Where it's used

AutomotiveTail-light and headlamp housings, wire harness splices
BatteriesCell tabs and busbars in electric-car and phone batteries
MedicalFilters, IV parts, sealed device housings
PackagingBlister packs, coffee capsule lids, tube sealing
Consumer goodsToys, toothbrushes, phone parts, nonwoven masks and wipes

How to recognize it

Two molded plastic halves joined with no screws, clips or glue, and a thin seam that cannot be opened.
Sometimes a fine line of squeezed-out plastic along the seam.
On metal wire splices or battery tabs, a small flattened area with a textured, knurled pattern left by the horn.

Compare with

Resistance spot welding7.2also joins overlapping parts at small areas, but with current and melting, for thicker steel sheets.Brazing and soldering7.3joins wires and tabs with a filler; ultrasonic welding needs none and adds no heat to the rest of the part.Friction stir welding7.6another friction-based method, but with a rotating tool for thick aluminum plates.

Watch it

DuskLight · The parts of the machine, animated

The Weld Nugget · How vibration becomes heat at the joint

Branson Ultrasonics · A welder working on an automated line

HerrmannUltrasonics · Many real products made this way

See real photos

Wikipedia: Ultrasonic welding — the parts of the welding stack and applications, with links to photos
Wikimedia Commons photo search

Remember

Vibration at 20,000 or more cycles per second turns into heat right at the joint.
Plastic joints melt at a small ridge; metal joints bond without melting by scrubbing sideways.
It is fast and clean, but limited to small joints and thin parts.